When products are manufactured in the form of material webs, such as printed products like labels or packaging, monitoring and/or automated quality assurance after printing is of considerable importance in order to check the printed product. In addition to automated monitoring, help with visual monitoring by an operator can also be provided here. In this type of quality assurance, the material webs are passed through monitoring or inspection systems that take pictures of the material webs. These pictures can be checked by an operator or automatically. In order to guarantee that pictures are of a high quality, the material webs may be lit for the pictures using corresponding devices.
In one particular type of inspection, what is referred to as multi-inspection, an attempt is made to capture the entire material web (100% inspection) with two types of lighting. However, more than two, in other words three, four or five types of lighting may also be used. The conditional alternating or staggered taking of pictures with different types of lighting and picture parameters means, for example, that two sequences of pictures with different information can be assessed. However, an offset is produced here between the taking of individual pictures and the sequences of pictures in the direction of movement of the material web which, for example, have been taken with incident lighting and transmitted light lighting. The effects of the offset may be greater the faster the web speed, the higher the resolution of the matrix chip and the greater the delay between the taking of pictures and sequences of pictures. In known inspection systems, an attempt is made to compensate for this offset by complex means using software and corresponding computer power because the sequences of pictures in the two (or more) types of inspection need to be laid exactly on top of one another for assessment in order to give the same referencing or the same reference (in the case of inspection errors) to the moved object. On the other hand, it is also possible to ignore a slight offset, but this is unsatisfactory. In other systems with software compensation, an algorithm is usually used to stabilize the pictures in which identical patterns are laid on top of one another after pattern matching using registration zones. However, the disadvantage with different lighting and therefore also other picture information is that it is very difficult to find identical patterns. The same lighting is actually a prerequisite here. To compensate for this using software is, on the one hand, very complex and, on the other, sometimes completely impossible or unsatisfactory in multi-inspection systems. The characteristics of the material web, which may be transparent, translucent, opaque or a mixture, may also result in unsatisfactory software compensation.
The aim of the present invention is therefore to provide a device and a method which overcome the disadvantages of software compensation.